This paper reports on an investigation of newly developed electrically conductive aggregates (ECAs) through the semi-dry mixing method and the pelletization technique. Carbon fiber and carbon black were incorporated into the aggregates as conductive fillers, while ordinary Portland cement and fly ash were used as matrix materials. The effects of carbon fiber and/or carbon black dosages on the electrical resistivity, water absorption and crushing strength of ECAs were studied. For ECAs with carbon fiber only, the threshold percolation of carbon fiber was identified to be 1.0% by volume. The ECAs with 1.0% carbon fiber exhibited 3.4 Ω·m electrical resistivity, 13.08% water absorption and 1.57 MPa crushing strength. Moreover, the effect of carbon black content was investigated when the content of carbon fiber was kept at 0.5 vol.%. The threshold percolation of carbon black for ECAs with 0.5 vol.% carbon fiber was 2.0% by weight. These ECAs possessed 7.34 Ω·m resistivity, 24.41% water absorption and 0.95 MPa crushing strength. Scanning electron microscope was employed to study the conductive network formed by two conductive components, which helped to illustrate the conductive mechanism of carbon fiber and carbon black inside the ECAs.